Alocasia sakonakhonensis (Araceae), a new species from northeastern Thailand

Abstract Alocasia sakonakhonensis Chatan & Promprom (Araceae), a new species from northeastern Thailand, is described and illustrated. It is clearly different from other previously known species by leaves, spathe, ovary, sterile interstice, synandria, and appendix. Color illustrations, and a distribution map are provided, as well as comparative morphological characters about its similar species. The preliminary conservation status assessment of the new species was presented.

During our floristic surveys in the Northeast of Thailand done by the authors in 2019-2022, many exciting specimens belonging to the family Araceae were found.Also, Alocasia specimens were collected to study their morphology.After carefully studying their morphological characters, the authors found that they were not matched to other previously known Alocasia species, representing an undescribed species.We describe and illustrate another new Alocasia species from this area in the present paper, namely Alocasia sakonakhonensis Chatan & Promprom.In order to enhance our understanding, pertinent references (such as Boyce, 2007;Boyce, 2008;Boyce & Sookchaloem, 2012;Hay & Wise, 1991;Hay, 1996;Hay, 1998;Hay, 1999;Li & Boyce, 2010;Nicolson, 1987, etc.) were consulted.Morphological measurements were conducted on fully developed specimens of mature flowering plants in their natural habitat.This was supplemented by measurements taken from herbarium specimens.

| MATERIAL S AND ME THODS
Upon a comprehensive examination of these specimens, they revealed characteristics that did not align with any previously documented taxon.Subsequently, a new Alocasia species was identified and is herein described and illustrated.Detailed morphological descriptions and a species key pertinent to Alocasia in Thailand are provided.The terminology concerning plant attributes adheres to the conventions laid out by Boyce (2008) and Boyce and Sookchaloem (2012).Furthermore, an evaluation of conservation status was executed in accordance with the Guidelines for Using the IUCN Red List Categories and Criteria, Version 16 (IUCN Standards and Petitions Committee, 2024).

| Phenology
The flowering period is from May to June, and the fruiting period is June to July.

| Distribution
Until now, Alocasia sakonakhonensis Chatan & Promprom has been considered endemic to Thailand.It has been found in only one population in the Phu Phan mountain range, located in Sakon Nakhon province, northeastern Thailand.(Figure 3).

| Ecology
This new species grows in shaded areas in dry evergreen forests at an elevation of 300-320 m.The plants grow in moist places on limestone, especially along streams.

| Vernacular name
Bon Pa.

| Etymology
The specific epithet of Alocasia sakonakhonensis refers to the type locality, the Sakon Nakhon province in the northeast of Thailand.

| Preliminary conservation status
Alocasia sakonakhonensis has not been recorded or described so far.This species grew in two small populations under the forest canopy in Sakon Nakhon province at an altitude of 300-380 m.The plant is estimated to have fewer than 2500 mature individuals, with each subpopulation consisting of fewer than 250 mature individuals.Therefore, it should be considered as "Endangered (EN)," according to IUCN criteria C(C2ai) (IUCN Standards and Petitions Committee, 2024).

| Taxonomic notes
Alocasia sakonakhonensis represents a non-pachycaul herbaceous taxon.It shares vegetative similarities with A. acuminata, characterized by non-seasonal dormancy, the absence of pachycaul and basal branching, short internodes, and peltate, bright green leaf blades.
However, in terms of morphology, the new species can be readily distinguished from the latter.This distinction is evident in the moderately green color of the petiole and peduncle, which feature numerous dirty mottled dark brown lines or areas measuring 1-3 mm (as opposed to the bright green color without such lines).Additionally, the length of these organs-including the peduncle, spathe, spathe limb, spadix, sterile interstice, and appendix-differs in A. sakonakhonensis, being more extended than those found in A. acuminata (measuring 25-28 cm, 13.0-14.5cm, 9.5-10.5 cm, 11-12 cm, 23-25 cm, and 5.0-5.2cm in length, respectively).The stigma of A. sakonakhonensis is distinctly 2-4 lobed, very rarely unlobed (in contrast to A. acuminata, which is very slightly unlobed).Furthermore, the petiolar sheaths of A. sakonakhonensis are open, unlike those of A. acuminata, which are closed.In A. acuminata, the appendix exhibits a dull yellow-brown color (compared to the white color in A. acuminata).Additional details regarding the morphological distinctions between A. sakonakhonensis and A. acuminata are presented in Table 1.Below is a modified key to the non-pachycaul herbaceous taxa of Alocasia in Thailand, adapted from Boyce and Sookchaloem (2012).
Key to non-pachycaul herb taxa of Alocasia in Thailand.

Field
trips were conducted in the Phu Phan mountain range located in Sakon Nakhon province, situated in the Northeast of Thailand, spanning from 2019 to 2022.During these trips, plant specimens were meticulously collected for the purpose of study.The type specimens and herbarium specimens belonging to morphologically related species within the Alocasia genus (obtained from Royal Botanic Gardens, Kew (K)), Forest Herbarium, Department of National Parks, Wildlife and Plant Conservation (BKF) and Bangkok Herbarium, Plant Varieties Protection Office (BK) were meticulously examined.Additionally, digitized images of specimens available on the JSTOR Global Plant database (https:// plants.jstor.org/ ) were also analyzed.